1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a mobility method performed by UE in a wireless communication system and an apparatus supporting the same.
2. Related Art
3rd generation partnership project (3GPP) Long Term Evolution (LTE) is an improved version of a Universal Mobile Telecommunication System (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) in a downlink, and uses Single Carrier-Frequency Division Multiple Access (SC-FDMA) in an uplink. The 3GPP LTE employs Multiple Input Multiple Output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-Advanced (LTE-A) that is an evolution of the 3GPP LTE.
A micro cell, a femto cell, a pico cell, etc. having small service regions may be installed at a specific location of a macro cell having wide coverage.
Since UE represented as a mobile device moves, a cell having low quality of service now being provided or capable of providing better service may be detected. For this reason, the UE may move to a new cell, which is the execution of the mobility of the UE.
Frequency that UE performs mobility may vary according to circumstances because each cell has fixed coverage and the UE moves on a wireless communication system at a variable speed. In order to support the mobility of UE by taking the mobility state of the UE into consideration, Mobility State Estimation (MSE) and a method of scaling a mobility parameter have been supported. The UE may determine whether or not to report measurement results based on the MSE and the method, and the mobility of the UE may be performed based on the reported measurement results. Accordingly, the MSE and the scaling of a mobility parameter are closely related to the execution of the mobility of the UE.
Meanwhile, a wireless communication environment in which a macro cell and small cells coexist may be provided. Such a wireless communication environment may be called a heterogeneous network. In a heterogeneous network environment, several small cells may be present in the coverage of a macro cell. In such an environment, if UE moves based on a measurement report based on the existing MSE and mobility parameter scaling, a problem, such as a Radio Link Failure (RLF) or a handover failure, may occur. Accordingly, there is a need to provide a mobility method that can be adaptively executed depending on the mobility state of UE and the type of target cell in a communication environment in which a macro cell and small cells coexist.